This is a weird phenomena that I've noticed ever since I started going low-carb. Any time I break diet and eat higher carb (basically, anything outside fruit and vegetables), I get incredibly thirsty. Like insanely drink half-a-gallon-of-fluids-and-feel-full-but-am-still-thirsty thirsty. When I go back to low-carb, this goes away, but it's bizarre while it's happening. Has anyone else experienced this phenomena? Is it related to the insulin spike?

From what I've experience if you have a lot of adrenal stress (can be from life, staying up late, lifting really heavy a lot, etc.) your mineralocorticoids probably get thrown out of whack b/c high production stress for adrenaline and glucocorticoids like cortisol take priority. Thus, decreased output of hormones like aldosterone (mineralocorticoids) which regulate K/Na and water reabsorption.

This leads to frequent urination and "unquenchable" thrist.

The other guess I would probably say is something along the lines of hypothalmic or anterior pituitary dysfunction along the same lines. Antidiuretic hormone would probably be the other.... but that would more involve LOTS of urination (aka diabetes insipidus..).

Hypothalamus regulates thirst so if anything in the HPA axis (hypothalamus, pituitary, adrenal) gets jacked up then there's gonna be problems

It could be adrenal-related as Steven mentions, or it could just be a massive need for water for all the glycogen you're taking in on the rare occasion (is it three molecules of H20 for every glycogen, if I remember correctly?).

I'm going with muscle glycogen replenshishment into muscles and taking more water from the body in the process...hence it makes you thristy to replenish......this is why BB getting ready for a show don't drink a ton of water during a "carb load" day because they want the water under the skin to be drawn into the muscle for a more "ripped" look.

You won't be as thristy after a day or two of high carbs....as the muscles will be stocked.

It's only a short period--during, and maybe twenty-four hours after, so it sounds like it's just water being replaced in muscles.

This is a fairly recent phenomena (maybe past six months). I've been lifting consistently for almost a year now--maybe I've just reached the point where I've build enough muscle mass that I notice the glycogen-replenishment effect.

I'm going with muscle glycogen replenshishment into muscles and taking more water from the body in the process...hence it makes you thristy to replenish......this is why BB getting ready for a show don't drink a ton of water during a "carb load" day because they want the water under the skin to be drawn into the muscle for a more "ripped" look.

You won't be as thristy after a day or two of high carbs....as the muscles will be stocked.

I'm with this as well. Have been low carb Paleo for awhile and on the days I have starchy carbs I get very thirsty and my mouth gets dry. If I drink water I get bloated. But if it's a dry carb load I get a little more "ripped".

The upper limit for glycogen storage in a well conditioned athlete is ~15g/kg BW so assuming you weigh ~60kg your poor starving muscles might be trying to pack in up to 900g of glycogen (most likely it'll be ~600g) along with 3g of water per gram of glycogen. This means you might need to drink up to 2.7L of water in addition to your normal fluid requirements plus a little extra if your high carb foods contain a lot of salt eg. bread. Then if you take into account the fact that glycogen resynthesis is pretty slow at ~5-7%/hour meaning it might take 20+ hours before your glycogen stores are topped up to capacity it's easy to see why you might feel thirsty for 24 hours or so after your high carb meal.
Of course you could avoid all this and improve your performance by eating more carbs more often.